(i) Field of the Invention
This invention relates to the continuous production of lead alloy strip and, more particularly, relates to the continuous high-speed extrusion of lead alloy strip for use as positive and negative electrodes of a lead-acid battery. The strip has a highly controlled microstructure which increases battery life by reducing the rate of vertical growth in the positive plate and reduces the rate of corrosion when compared with plates produced by other continuous processes. By reducing growth in the positive plate and by minimizing and forming a desired strip profile (e.g. by varying or tapering thickness from top to bottom of the strip) for the positive and negative alloy strip, the mass of both the positive and negative plates can be reduced, thereby reducing the overall weight and cost of batteries.
(ii) Description of the Related Art
In the production of lead-acid batteries there are several methods to produce the positive and negative grids used in the battery. In the continuous production of grids for lead-acid batteries these processes are limited to the production of either a rolled or cast strip which is punched or expanded by either reciprocating or rotary expansion processes or to the direct casting of grids, such as by the Concast(trademark) process.
The production of lead alloy strip for use as positive battery plates having limited plate growth is extremely important because plate growth can cause cell short circuits.
This is a leading factor in shortening battery life in batteries made by continuous processes. The strip produced by conventional method such as rolling or continuous casting typically have a highly heterogenous microstructure with non-uniform grain sizes and shapes leading to undesirable plate growth and to corrosive attack from the battery electrolyte.
The production of strip for negative plates also is commonly done by either continuous cast or rolling processes. The negative electrode is not subjected to corrosive attack due to the electrochemical characteristics of the electrode, and therefore the main focus of the negative strip is reducing the weight of the negative plate, while maintaining adequate conductivity. This is done by simply making the strip thinner; however if the lug is too thin there are problems in battery manufacturing, related to melting of lugs into the top lead.
The extrusion of lead and lead alloys to provide a protective sheath on submersible cables to protect the cables from the corrosive effects of seawater, by means of extruding machines, has been done extensively in the past in the cable industry. Electrical cable is passed through the machine and a layer of lead alloy tube is extruded onto the cable. H. F. Sandelin is a world leader in the production of this type of machine. Other manufacturers include Pirelli, which produced a machine which is similar to the older Henley Extruder(trademark) in utilizing a large horizontal screw. This type of apparatus has problems with alloy segregation and contamination of the screw.
U.S. Pat. No. 4,332,629 describes the production of lead-antimony alloy strip by ram-press extrusion. This process is limited to certain thicknesses and aspect ratios. Also, the process has limitations on production speed, this patent disclosing production rates of 6-10 ft/min. (1.9-3.2 kg/min.). Strip produced by ram press extrusion has had negative results regarding corrosion and grid growth in laboratory testing. Also this process does not provide control of microstructure and grain size and is limited to alloy selection.
The objective of the present invention is to provide an extrusion method and apparatus that can economically produce superior extruded lead alloy strip of a desired profile for the production of positive and negative battery plates that are resistant to both vertical grid growth and weight loss through corrosion in lead-acid batteries. The strip is produced at a speed that is competitive with continuous cast and rolled strip, the extruded strip having superior qualities in any of the following areas; corrosion induced growth, corrosion weight loss, shape, grid weight, cost, and automation compared to conventional technologies.
The preferred use of the invention is in the production of lead-alloy strip to be coiled for use in a continuous battery manufacturing line. This strip can be used to produce battery mesh by continuous reciprocating expansion of the strip into expanded mesh or by continuous rotary expansion of the strip into expanded mesh, such as disclosed in U.S. Pat. No. 4,315,356 issued Feb. 16, 1982, U.S. Pat. No. 4,291,443 issued Sep. 29, 1981, U.S. Pat. No. 4,297,866 issued Nov. 3, 1981, U.S. Pat. No. 5,462,109 issued Oct. 31, 1995, and U.S. Pat. No. 5,896,635 issued Apr. 27, 1999 to Cominco Ltd., incorporated herein by reference. The expanded mesh is then pasted and divided into individual battery plates that can be placed in a battery.
The minimum and maximum grain sizes of the extruded strip will vary with the thickness of the strip but can be controlled by means of rapid cooling with water spray after the strip exits the extruder die. The microstructure of extruded alloy strip is homogenous, stable and can be easily controlled through machine parameter adjustments. With the correct choice of alloy and grain size, the vertical growth of positive grids in the lead-acid battery, made with extruded strip, can be greatly reduced. When compared with the current continuous processes for strip production, the growth of the positive grid is reduced by 50-75% in common laboratory testing. Weight loss caused by corrosion of the grid is similar to that of grids produced from continuously cast strip, and is less than that of rolled strip or bookmold grids. In extrusion, the strip can have different grid thicknesses over the width of the strip by control of the strip profile. This allows plates to be made with very thin wires, while still having a lug thickness sufficient to overcome manufacturing problems associated with thinner lugs. This leads to a significant weight savings in the negative plate and reduces the overall weight and cost of the battery. It has been found that by modifying the die block to allow for strip production rather than tube production, planar high-quality lead alloy strip can be produced of a desired profile. By introducing a novel strip cooling system, which preferably is a water spray system outside of the extruder dieblock, the strip can be optimized in alloy composition, grain size and thickness for fabrication of battery plates for use in lead-acid batteries.
The main advantage of the extrusion strip production method is the absolute control over the grain size and grain structure of the material. This allows for the optimization of these parameters for reducing corrosion, limiting corrosion induced growth, increasing strength, and manipulation of the aging process of the alloy.
Specifically, there are eight areas for optimizing strip and the resultant grids in a battery.
1. Grain Size: Extrusion offers the possibility of controlling the actual grain size of the final product over a wide range, from 20 microns to 500 microns. It should be noted that the minimum grain size will be further influenced by strip thickness and strip alloy composition. While it is possible to produce grain sizes anywhere in this range by modifying the cooling distance from the dieblock exit or the cooling rate, it should be noted that for battery performance it is preferred that the grain size for positive electrodes be in the range of 100-500 microns, most preferably in the range 100-300 microns. This is because at very small grain sizes of less than 100 microns, e.g. 20-100 microns, the grain boundary path for corrosive attack is almost straight through the material along the boundaries of very many grains. At very large grain sizes of greater than 100 microns, the path is also quite straight along the path of only one or two grains. Negative electrodes, however, are not subjected to corrosive attack and small grain sizes down to 10-30 microns are acceptable.
2. Grain Structure: Extruded strip produces ahomogenous, equiaxed grain structure that is unlike any other strip production method. In conventional continuous casting the grains are columnar and very long. This can lead to a very straight path for corrosion along grain boundaries through the entire thickness of the strip, leading to significant grid growth in the battery. Rolled strip has a very heterogeneous, stratified structure with significant defect structure throughout the thickness of the strip. While this structure does impart high strength, it also allows significant corrosive attack on the strip, leading to very high weight loss due to corrosive attack at the defect sites. The deformed grains simply are peeled away, layer by layer, by the corrosive attack. With the extruded strip, the equiaxed, homogenous structure, with an optimized grain size, having about 6-10 grains (100-300 micron grain size) through the thickness of the material, provides a very limited defect structure and also presents a long and winding path for corrosive attack on the grain boundaries of positive electrodes. In order for corrosive product to penetrate the strip and proceed through the thickness of the strip, the grain boundary path would be quite long and would slow the process down significantly.
3. Strip Tolerance: The extruded strip can be produced with extremely accurate physical dimensions with strip thickness tolerances of +/xe2x88x920.025 mm. There is no need to trim the edges of the resultant strip as it can be produced to the exact width that is required without affecting the properties of the strip near the edges.
4. Alloy: Extrusion offers a wide range of possible alloys, similar to that of continuous casting or rolled strip. There are some elements that should be avoided in the alloy composition for extruded strip. These include aluminum, bismuth and sulphur, which will preferentially deposit on the extrusion screw, increasing the friction on the screw as it transports the lead to the die block. At a certain point the friction forces will cause the lead to stop moving in the screw housing, leading to down time for maintenance and cleaning of the screw.
5. Lead Placement (Strip Profile): With extruded strip, the lead passes through a machined die in order to achieve the strip profile necessary (i.e. strip width and thickness). With this in mind the strip can be profiled such that there is sufficient lead thickness for lug welding and conductivity; however the wire thickness can be decreased in order to reduce the weight of either a positive or negative plate. It is most useful in the negative plate, since the thickness of the strip can be significantly reduced between the top and bottom borders of the grid. This can also be useful in relocation of metal in the positive grid for conductivity reasons.
6. Porosity of the Strip: It is well known that the extruded lead product has zero porosity. This has been extremely important in the submarine cable industry, as any level of porosity could lead to seawater intrusion and cable failure, at a huge expense to the owner/operator of the cable. Thus many tests have been done on the extruded product to prove that there is absolutely no porosity, which is desirable for battery strip since any defects such as porosity can lead to aggressive corrosion attack. In continuous cast strip the level of porosity is quite low, although there is some minor levels, depending on alloy composition, casting speed, etc. Rolled product can have significantly high levels of porosity due to inclusions and impurities that are trapped in the product during the rolling process. Book mold grids, while not continuous, have very high levels of porosity that can lead to plate failures due to very high weight loss through corrosion. The pores, which are defects in the product, lead to more surface area that is open to corrosion attack.
7. Grid Design: With the extruded strip the mechanical properties are quite impressive. There is a high initial tensile strength (which varies with alloy) and a very high elongation ( greater than 40%) before yield. This elongation stays high through the early aging process, which allows the use of high elongation tooling in a rotary expansion system. The use of higher elongation tooling allows the diamond design of battery grid mesh to be almost square, thereby bringing the SWD/LWD ratio close to 1. In the diamond the SWD is height from the top of the diamond to the bottom and the LWD is the width of the diamond. This will aid in reducing growth, as the material is physically stronger in the vertical direction with this type of geometry as compared to grids produced with conventional elongation tools. Conventional SWD/LWD ratios, as shown in FIG. 4, are much less than 1.
8. Aging of the Strip: It has been shown that both continuous cast and rolled strip product will suffer from overaging after a certain period of time, as shown in FIG. 5. This phenomenon, which is dependent on alloy composition and environmental conditions, significantly decreases the strength of the grid material, thereby increasing the possibility of grid growth. It has been shown that in standard 60xc2x0 C. aging tests, rolled strip will significantly overage after approximately 90 days. Continuous cast material, dependant on alloy composition, can extend this period. It has been found in extruded strip that the aging process continues for upwards of 130 days without any significant overaging. This does not seem to be dependant on the alloy composition, however there is some effect from the grain size, with the smaller grain sizes in the range of 10 to 30 microns having a lower overall strength reduction of approximately 10% after significant aging. In its broad aspect, the method of producing a lead alloy strip for battery electrodes comprises extruding a lead alloy through a die block to produce an extrusion having a desired shape and rapidly cooling the extrusion to acquire a lead alloy grain size in the range of about 10 to 300 microns. More particularly, the method comprises extruding the lead alloy in the shape of a substantially planar profile or in the shape of a tube extrusion, slitting and opening the tube, and rolling the opened tube into a planar strip prior to rapidly cooling the extrusion. The planar strip preferably is cooled under tension and the cooled wound strip into a coil. The method additionally comprises slitting and expanding the cooled planar strip into an expanded grid by rotary expansion or by reciprocating expansion, punching, machining, waterjet cutting, spark cutting or laser cutting. Expanded grids are particularly suited for use as a battery electrode, such as in lead-acid batteries.